Beta-alumina was first reported by Rankin and Merwin in 1916. However, extensive research on this material did not take place until fifty years later, when its use as a solid electrolyte for the sodium-sulfur battery was investigated. Sodium Beta-alumina exhibits the unique property of having a high sodium ion conductivity at 300.degree. C., which is the temperature at which the sodium-sulfur battery is operated.
Sodium Beta-alumina is a highly ion-conductive ceramic which can be represented by the chemical formula: Na.sub.2 O.multidot.xAl.sub.2 O.sub.3, where x can vary between 5 and 11. Sodium Beta-alumina powder is generally produced by reaction fusion of Al.sub.2 O.sub.3 powder with Na.sub.2 O in the form of a sodium salt, at 1,250.degree. C. to 1,550.degree. C., to form Na.sub.2 O.multidot.xAl.sub.2 O.sub.3, as described by Iijima, in U.S. Pat. No. 4,082,826. The sodium Beta-alumina powder is then formed into the desired electrolyte shape, and sintered at up to 1,750.degree. C. DeJonghe et al., in U.S. Pat. No. 3,959,022, reduced sintering temperatures by first forming a eutectic mixture of sodium-aluminum oxide, in which the atom ratio of sodium to aluminum is 0.54. This eutectic mixture is added to sodium Beta-alumina powder and the additive mixture is heated at about 1,600.degree. C., to achieve reactive liquid phase sintering.
The grain size and purity of sodium Beta-alumina are extremely important factors influencing its ion conductivity and performance as a solid electrolyte. The high temperature fusion method of formation, where Al and Na are reacted in the solid state at from 1,250.degree. C. to about 1,550.degree. C., generally produces undesirable grain growth and purity problems.
The grains produced by this prior heat reactive method, generally have a random orientation and average dimensions of between about 1 to 3 microns. This type of structure allows grain boundary blockage of adjacent conduction layers, causing long ion migration paths and reducing the total conductivity of the material. During final sintering, grain growth increases even more. What is needed is an improved method of producing sodium Beta-alumina which will provide small grains of high purity, and increased sodium ion conductivity.